High performance phosphate green fluorescent powder and its method of preparation

ABSTRACT

This invention relates to a novel phosphate based green phosphor represented by the formula (Ln 1-x-y  Ce x  Tb y ) PO 4  ·wM where Ln refers to one or more than one of La, Gd, Y and M refers to one or more than one oxide of B 2  O 3 , Al 2  O 3 , In 2  O 3 , ZrO 2 , Nb 2  O 5  and TiO 2 . Requisite materials are processed through blending, prefiring, firing and post-firing treatments to form the high performance phosphate based green phosphor. The processing steps of this invention effectively obviate the difficulties as associated with the prior technology, substantially improve the phosphor brightness, thermal stability, enlarge the specific surface of the phosphor, and reduce the phosphor consumption in screen coating, thereby reducing the picture tube manufacturing cost and eventually yielding the novel high performance phosphate based green phosphor.

This invention relates to a novel phosphate based green phosphor and itsmethod of preparation and particularly, it relates to a high performancephosphate based green phosphor and its method of preparation through theintroduction of one or more than one oxides of either the boron groupelements or the refractory elements.

BACKGROUND

Triphosphor or three primary color phosphor is prepared by blendingthree color phosphors, including the red phosphor giving the mainemission peak at the wavelength of 611 nm, the green phosphor at 544 nm,and the blue phosphor at 450 nm when excited by ultraviolet light of thewavelength of 253.7 nm, the brightness being mainly contributed by thegreen. In 1976, the Philips Electronics of Netherlands developed themagnesium polyaluminate based green phosphor coactivated by Ce and Tb,making an excellent contribution to the development of triphosphorfluorescent lamps (EP 1458700). However, the very high firingtemperature in excess of 1500° C. and prolonged heating as required bysuch green phosphor resulted in hard caking of the calcined mass thatwas difficult to undergo post-firing treatment. Low material yield andshort service life of the refractories used led to a high productioncost. Therefore, active studies and increasing commercial applicationsare being directed to the phosphate based green phosphor coactivated byCe and Tb.

Phosphate based green phosphors were reported fairly early. In 1968, R.C. Ropp of Westinghouse Electric Corp. studied phosphors made by usingLaPO₄, YPO₄, and GdPO₄ as matrixes and Eu³⁺, Ce³⁺, Tb³⁺, Sm³⁺, Tm³⁺, andDy³⁺ as activators [J. Electrochem. Soc., 115, 841 (1968)]. In 1971, M.V. Hoffman of General Electric studied a phosphor of composition(La_(1-x-y) Ce_(x) Tb_(y)) PO₄ [J. Electrochem. Soc., 118, 1508 (1971)].In 1979, Mitsubishi Electric Corp. of Japan reported a green phosphor ofcomposition (La_(1-x-y-p-q) Gd_(x) Y_(y) Ce_(p) Tb_(q)) PO₄ [JP54-56086] and 0.15 was considered as the optimized value of p in thecomposition formula, any further increase in p would bring about adecrease of brightness. All phosphate based green phosphors reportedpreviously suffered from low brightness and could hardly be used incommercial production.

In 1982, Nichia Chemical Industries Ltd of Japan reported on thephosphor of composition (Ce_(1-x-y) La_(x) Tb_(y)) PO₄ [JP57-23674],where 0.1<(x+y)<0.4, 0.05<y<0.3. The phosphor was prepared by the wellknown dry blending method or the wet precipitation method. In thatinvention, phosphor brightness was increased by displacing small amountof Ce by La and high firing temperature >1500° C. was also obviated. Butthe Ce³⁺ concentration in the phosphor was relatively high (1-x-y=0.7was considered as optimized) and Ce³⁺ had a strong tendency of beingoxidized making the phosphor less stable, less durable, easier to decayin brightness and more vulnerable to ultraviolet emission. When dryblending process was used for its preparation, the phosphor was found toagglomerate and stick to the wall since (NH₄)₂ HPO₄ was hygroscopic.This made it difficult to obtain a homogenous mixture, resulting inlocal excess or insufficiency of P/RE (phosphorus and rare earth ratio).It was therefore difficult to obtain high performance phosphor. Whenusing the wet precipitation process, the phosphate obtained was small inparticle size, making its washing and filtration difficult and resultingin formation of cakes which needs disintegration. Moreover, the particlesize distribution of the green phosphor made was rather wide due to thepresence of a large amount of fine particles. The quality of the coatfilm was, therefore, not satisfactory and its luminous efficiency waslow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the effects of cerium addition to the rare earth of thephosphor.

FIG. 2 shows the effects of terbium addition to the rare earth of thephosphor.

FIG. 3 shows the effect of phosphate to rare earth ratio on relativebrightness.

DETAILED DESCRIPTION

This invention aims at offering a novel rare earth phosphate based greenphosphor and its method of preparation. The composition of the rareearth phosphate based green phosphor can be represented by

    (Ln.sub.1-x-y Ce.sub.x Tb.sub.y) PO.sub.4 ·wM

where Ln stands for one or more than one of the rare earth elements suchas La, Y, and Gd;

M stands for one or more than one of the boron group oxides orrefractory oxides such as B₂ O₃, Al₂ O₃, In₂ O₃, ZrO₂, Nb₂ O₅, and TiO₂;

    and 0.05≦x≦0.7, 0.05≦y≦0.4 1×10.sup.-2 ≦w≦1×10.sup.-1.

The processing steps for the preparation of the rare earth phosphatebased green phosphor of this invention are outlined below:

1. Material Preparation

(NH₄)₂ HPO₄ as bought is first dried at low temperature for 5 hours, thecomponent M of the amount calculated according to the compositionformula given above is then added along with an appropriate amount offlux Li₃ PO₄. After blending for a few hours, the material obtained isreferred to as material A.

Ln₂ O₃, CeO₂, and Tb₄ O₇ of amounts based on the composition formulaabove are weighed out, water is added to them to form a slurry, andnitric or hydrochloric acid is added to dissolve the oxides to a finalpH of 1-3. The rare earth concentration in the solution should be about100 g/l of oxides. Then hot oxalic acid solution of concentration notexceeding 10% is added while stirring to form the rare earth oxalateprecipitate. The precipitate is thoroughly washed with deionized water,filtered and dried. Firing at 1100° C. for 1-3 hours gives (Ln_(1-x-y)Ce_(x) Tb_(y))₂ O₃ which is referred to as material B.

2. Blending

Material A and material B in calculated ratio are placed into a V-typeblender, in which the mole ratio of phosphate radical in material A toRE in material B is (1-1.2):1 and the amount of flux Li₃ PO₄ should beabout 0.3% by wt of rare earth phosphate. Blending for more than 5 hoursgives the AB mixture.

3. Prefiring

The thoroughly mixed AB mixture are placed into a quartz or aluminacrucible and then it is prefired in an oxidizing atmosphere at about800° C. for 2-3 hours. After being furnace cooled, the prefired materialis discharged.

4. Firing

The prefired material is then crushed, pulverized, sieved with a 100mesh sieve, repacked into the crucible, and fired at 1100°-1150° C. in areducing atmosphere of a mixture of 1-5% by vol of hydrogen and 95-99%by vol of nitrogen for 1 to 2 hours. After being furnace cooled to atemperature below 200° C., the fired material is discharged.

5. Post-firing Treatment

The fired material is placed into a ball mill and disintegrated withglass or plastic balls of diameter 2 to 5 mm for 1 to 2 hours, the ratioof feed:water:balls=1:1:1 by wt. After sieving with a 450 mesh sieve,the fine pulp is washed with deionized water 5 times and then filtered.The filter cake is then charged into a drying oven and after drying, itis further sieved with a 100 mesh sieve. After quality inspection, thisis the phosphate based green phosphor of this invention.

Compared with prior arts, in the novel rare earth phosphate based greenphosphor of this invention, the introduction of one or more than oneoxide of boron group elements or refractory elements into rare earthphosphates effectively depresses the oxidation of Ce³⁺ and improves thethermal stability of the phosphor. High brightness and high performancerare earth phosphate based green phosphor is thus obtained. Furthermore,the addition of oxides of boron group elements or refractory elementsimproves the crystal growth of particles, weakening the tendency of theparticles to agglomerate. The specific surface of the phosphor particlesis increased and thus the phosphor consumption in screen coating isdecreased, thereby the lamp tube manufacturing cost is reduced.

Improvements are also made in this invention over the existing method ofphosphor preparation. The (NH₄)₂ HPO₄ as bought is previously dried atlow temperature before blending with small amounts of additive and flux.The rare earth oxides such as coprecipitated (La, Ce, Tb)₂ O₃ which maypick up moisture from air are prefired at high temperature, thuseliminating the troubles of agglomeration and sticking to the inner wallof the blender. Such measures are believed to be more rationalized andhave been proved by practice to be very effective.

Before going into detailed discussions on the various factors affectingthe performance of the green phosphor, it is appropriate to have theterms, relative brightness and thermal stability, well defined below:

The relative brightness of phosphor refers to the ratio of thebrightness of the phosphor sample when excited by the ultravioletemission of 253.7 nm to the brightness of the standard sample understrictly identical excitation conditions. ##EQU1##

The thermal stability of the phosphor refers to the ratio of thebrightness of the phosphor sample measured after baking at 600° C. forhalf an hour and cooling to room temperature to the brightness measuredwithout the baking treatment. ##EQU2##

The accompanying figures will help the elucidation of the functions ofCe, Tb and PO₄ ³¹ in the rare earth phosphate based green phosphor ofthis invention.

FIG. 1 shows the effects of cerium addition to the rare earth of thegreen phosphor on relative brightness, thermal stability, and specificsurface of the phosphor. As indicated in this figure, the brightness ofthe phosphor increases with an increase in Ce addition in the initialstage, but rather slowly when Ce addition exceeds 0.2 atomic ratio andbegins to drop down when Ce addition exceeds 0.5 atomic ratio. Both thethermal stability and specific surface decrease as Ce additionincreases.

With an increase in Ce content, the energy absorbed upon excitation by253.7 nm ultraviolet emission increases and, therefore, the relativebrightness of the phosphor is improved. But, at the same time, theprobability of Ce³⁺ being oxidized to the tetravalent state alsoincreases. This is especially true when the phosphor is strongly heated,thus causing the phosphor to turn grey or brown and its brightness todrop rapidly. In order to prepare phosphor with high brightness, goodthermal stability, and large specific surface, the Ce addition in atomicratio should preferably range from 0.2 to 0.5.

FIG. 2 shows the effects of terbium addition to the rare earth of thephosphor on relative brightness, thermal stability and specific surfaceof the phosphor. As indicated in this figure, the phosphor brightnessincreases with an increase of Tb addition, but when Tb addition exceeds0.3 atomic ratio, on the contrary its brightness begins to drop. As isthe case with Ce addition, both the thermal stability and specificsurface decrease with an increase in Tb addition. The Tb addition inatomic ratio should preferably range from 0.1 to 0.3.

FIG. 3 shows the effect of phosphate to rare earth ratio on the relativebrightness of the phosphor. As indicated in this figure, when PO₄ ³⁺/RE³⁺ <0.8, the relative brightness of the phosphor is below 50%. It istherefore necessary to provide sufficient phosphate radical to ensurewell developed crystal particles. But excessive phosphate radical notonly reduces furnace charge, but also brings about some reduction of thephosphor brightness. The phosphate to rare earth mole ratio shouldpreferably be (1-1.2):1.

EXAMPLES

The following examples serve to illustrate the effects on thebrightness, thermal stability and specific surface of the rare earthphosphate based green phosphor and the preferable ranges of the variousadditions.

These examples are given for demonstration only. The scope of protectionof this invention will be further embodied in the paragraphs underclaims.

EXAMPLE 1

69.30 g of previously dried (NH₄)₂ HPO₄, 0.35 g of flux Li₃ PO₄, and0.51 g of Al₂ O₃ were weighted out and thoroughly blended. This resultedin material A.

According to the requirement that the phosphate radical contained inmaterial A and the rare earth contained in material B should comply withthe preferable mole ratio of (1-1.2):1, 40.75 g of La₂ O₃, 27.52 g ofCeO₂, and 16.80 g of Tb₄ O₇ were weighed out, dissolved in conc. HCl ornitric acid of concentration above 60% to a final pH of 1-3, and dilutedwith deionized water so that the rare earth concentration in thesolution was 100 g/l of oxides. The solution was heated to about 60° C.and warm oxalic acid solution of concentration not exceeding 10% wasadded while starring to form rare earth oxalate precipitate. Theprecipitate was washed with warm deionized water 5 times and thenfiltered. The filter cake, after being dried, was charged into acrucible and calcined at 1100° C. for 3 hours. The rare earth oxide(La₀.5 Ce₀.32 Tb₀.18)₂ O₃ thus obtained formed the material B.

Material A and material B were placed into a blender. After thoroughblending for more than 5 hours, the mixture was charged into a crucible,prefired at 800° C. for 2-3 hours, cooled and discharged. The refiredmaterial was then crushed, sieved with a 100 mesh sieve, recharged intoa quartz or alumina crucible, and fed into the furnace. The furnace wasfirst purged with high purity nitrogen until the air within it wascompletely displaced, and fed continuously with a mixed gas of 1-5% byvolume of hydrogen and 99-95% by volume of nitrogen while heating. Whenthe temperature came up to 1150° C., this temperature was kept for 2hours. The furnace was then cooled to 400° C., at this moment hydrogenfeeding was stopped with the nitrogen gas streaming through the furnacetill the temperature was further decreased to 200° C. The fired materialwas transferred into a ball mill with glass or plastic balls of 2-5 mmin diameter. To the mill was also added deionized water with the ball,charge and water ratio equal to about 1:1:1 by wt. After 2-3 hours ballmilling, the pulp was wet sieved through a 450 mesh sieve, washed withdeionized water 5 times, filtered, and dried at 120° C. The rare earthphosphate based green phosphor of this invention was thus obtained.

EXAMPLES 2-8

In these examples all samples were prepared in a way identical withExample 1, except that the amount of Al₂ O₃ added to material A waschanged into 0.92 g, 1.33 g, 1.73 g, 2.14 g, 2.55 g, 5.10 g, and 10.20 grespectively.

The values of w in the formula (La₀.50 Ce₀.32 Tb₀.18) PO₄ ·wAl₂ O₃) forExamples 1 to 8 were, therefore, respectively 1×10⁻², 1.8×10⁻²,2.6×10⁻², 3.4×10⁻², 4.2×10⁻², 5.0×10⁻², 1.0×10⁻¹, 2×10⁻¹.

In the five tables that follow, the results of all the examples(Examples 1-25) are given together with the results of a control sampleand a so-called standard sample GP-1. The control sample was prepared inthe same way as in Example 1 except that no Al₂ O₃ was added, itscomposition can thus be represented by (Ln₀.50 Ce₀.32 Tb₀.18) PO₄. Thestandard sample GP-1 is a Japanese sample of advanced quality in theinternational market.

Measurement results for Examples 1-8 are tabulated in Table 1.

                  TABLE 1                                                         ______________________________________                                        Results of Examples 1 to 8                                                                  Relative   Specific   Thermal                                                 Brightness Surface    Stability                                 Sample No.    Br(%)      Sv(cm.sup.2 /g)                                                                          Br'(%)                                    ______________________________________                                        Example 1     100.8      2400       92.5                                      Example 2     101.1      2800       92.8                                      Example 3     101.9      3300       95.0                                      Example 4     104.0      3400       97.0                                      Example 5     104.2      3460       97.8                                      Example 6     104.4      3520       99.2                                      Example 7     101.7      3820       96.0                                      Example 8     100.9      4060       95.7                                      Control Sample                                                                              98.9       2200       92.2                                      Standard Sample GP-1                                                                        100        3200       98.1                                      ______________________________________                                    

Table 1 indicates that along with the increase in the amount of Al₂ O₃added, both the brightness and specific surface of the phosphor are todifferent extents increased, and also is improved its thermal stability.The phosphate based green phosphor prepared in accordance with thecomposition and method of preparation of this invention indicated betterbrightness by 1-2% and larger specific surface by 200-300 cm² /g ascompared with the Japanese sample GP-1.

EXAMPLES 9-14

In these examples, all samples were prepared in a way identical withExample 1, except that ZrO₂ was used in place of Al₂ O₃, its amountbeing respectively 0.62 g, 1.11 g, 1.60 g, 2.09 g, 2.58 g, 3.08 g. Thesamples of these examples are, therefore, represented by (La₀.50 Ce₀.32Tb₀.18) PO₄ ·w ZrO₂) where the respective w values are 1×10⁻², 1.8×10⁻²,2.6×10⁻², 3.4×10⁻², 4.2×10⁻², 5.0×10⁻². Measurement results for Examples9-14 are tabulated in Table 2.

                  TABLE 2                                                         ______________________________________                                        Results of Examples 9 to 14                                                               Relative    Specific  Thermal                                                 Brightness  Surface   Stability                                   Sample No.  Br(%)       Sv(cm.sup.2 /g)                                                                         Br'(%)                                      ______________________________________                                        Example 9   99.5        2380      94.6                                        Example 10  101.6       2560      96.7                                        Example 11  102.3       2600      96.8                                        Example 12  99.5        2710      96.5                                        Example 13  98.8        2830      96.0                                        Example 14  96.5        2950      94.0                                        Control Sample                                                                            98.5        2180      92.0                                        ______________________________________                                    

Table 2 indicates that with the addition of ZrO2, the phosphorbrightness, thermal stability and specific surface of the phosphor areall to different extents increased.

EXAMPLES 15-21

In these examples, all samples were prepared in a way identical withExample 1, except that B₂ O₃ was used in place of Al₂ O₃, its amountbeing respectively 0.35 g, 0.63 g, 0.91 g, 1.18 g, 1.46 g, 1.74 g, 3.48g. The samples of these examples are, therefore, as represented by(La₀.50 Ce₀.32 Tb₀.18) PO₄ ·w B₂ O₃ where the respective w values are1×10⁻², 1.8×10⁻², 2.6×10⁻², 3.4×10⁻², 4.2×10⁻², 5.0×10⁻², 1×10⁻¹.Measurement results for Examples 15-21 are tabulated in Table 3.

                  TABLE 3                                                         ______________________________________                                        Results of Examples 15 to 21                                                              Relative    Specific  Thermal                                                 Brightness  Surface   Stability                                   Sample No.  Br(%)       Sv(cm.sup.2 /g)                                                                         Br'(%)                                      ______________________________________                                        Example 15  102.1       2400      95.4                                        Example 16  103.5       2700      96.2                                        Example 17  104.5       3100      97.8                                        Example 18  104.5       3200      98.5                                        Example 19  104.6       3450      99.4                                        Example 20  104.5       3800      97.8                                        Example 21  103.2       4200      97.5                                        Control Sample                                                                            99.8        2320      94.5                                        ______________________________________                                    

Table 3 indicates that the relative brightness, thermal stability, andspecific surface of the phosphor are all substantially improved by theaddition of B₂ O₃.

EXAMPLE 22

Identical with Example 1 except that the phosphor composition isrepresented by (La₀.50 Ce₀.32 Tb₀.18) PO₄ ·1×10⁻² In₂ O₃.

EXAMPLE 23

Identical with Example 1 except that the phosphor composition isrepresented by (La₀.50 Ce₀.32 Tb₀.18) PO₄ ·1×10⁻² Nb₂ O₅.

EXAMPLE 24

Identical with Example 1 except that the phosphor composition isrepresented by (La₀.50 Ce₀.32 Tb₀.18) PO₄ ·1×10⁻² TiO₂

Measurement results for Examples 22 to 24 are tabulated in Table 4.

                  TABLE 4                                                         ______________________________________                                        Results of Examples 22 to 24                                                              Relative    Specific  Thermal                                                 Brightness  Surface   Stability                                   Sample No.  Br(%)       Sv(cm.sup.2 /g)                                                                         Br'(%)                                      ______________________________________                                        Example 22  102.1       3400      98.5                                        Example 23  101.3       3340      98.2                                        Example 24  101.1       3300      98.0                                        Control Sample                                                                            99.5        2400      95.2                                        ______________________________________                                    

Table 4 indicates that the addition of In₂ O₃, Nb₂ O₅, and TiO₂ bringsabout some improvement on the brightness, thermal stability, andspecific surface of the phosphor.

EXAMPLE 25

Identical with Example 6 except that further addition of B₂ O₃ was made.Thus, the phosphor composition is represented by (La₀.50 Ce₀.32 Tb₀.18)PO₄ ·5×10⁻² Al₂ O₃ ·5×10⁻² B₂ O₃.

Measurement results are given in Table 5 which indicates thatsimultaneous addition of both Al₂ O₃ and B₂ O₃ is beneficial inincreasing the brightness, thermal stability, and specific surface ofthe phosphor.

                  TABLE 5                                                         ______________________________________                                        Results of Examples 25                                                                    Relative    Specific  Thermal                                                 Brightness  Surface   Stability                                   Sample No.  Br(%)       Sv(cm.sup.2 /g)                                                                         Br'(%)                                      ______________________________________                                        Example 25  104.2       4150      99.5                                        Example 6   104.4       3520      99.2                                        Control Sample                                                                            98.9        2200      92.2                                        ______________________________________                                    

Beneficial effects of this invention is quite obvious. Comparing withsimilar products of advanced quality in international market, thephosphor prepared in accordance with this invention manifests superiorbrightness by 2% or above, superior thermal stability by 1% or above,and larger specific surface by 500 cm² /g or more.

We claim:
 1. A novel rare earth phosphate based green phosphor with acomposition represented by

    (Ln.sub.1-x-y Ce.sub.x Tb.sub.y) PO.sub.4 ·wM

where Ln is one or more elements selected from the group consisting ofLa, Y, and Gd; M is one or more oxides of the elements selected from thegroup consisting of Zr, Nb and Ti; and 0.05≦x≦0.7, 0.05≦y≦0.40,0.01≦w≦0.1.
 2. The green phosphor of claim 1 wherein said oxide iszirconium dioxide.
 3. The green phosphor of claim 1 wherein said oxideis niobium pentoxide.
 4. The green phosphor of claim 1 wherein saidoxide is titanium dioxide.